The present invention relates to a method for joining fastening elements such as studs on workpieces such as metal sheets, said method having the steps of preparing a joining surface of a fastening element and/or a joining surface of a workpiece and of bringing the fastening element and the workpiece together in such a manner that the fastening element is joined onto the workpiece, wherein, prior to the joining step, a cleaning step is carried out in order to clean the joining surface of the workpiece.
In addition, the present invention relates to an apparatus for joining fastening elements on workpieces, in particular for carrying out the abovementioned method, said apparatus having a fastening element holding device for holding a fastening element, wherein the fastening element holding device is displaceable along a first axis by means of a drive device.
One joining method of the abovementioned type is, for example, so-called stud welding. With said method, fastening elements, such as studs, nuts or metal strips, are welded at right angles onto a surface of a workpiece, such as a metal sheet, the method being possible with the workpiece being accessible on one side.
Stud welding is used extensively in the area of bodywork construction for motor vehicles, the fastening elements joined on workpieces, such as body sheets, in this way serving, for example, as anchors for clips to which, in their turn, cable looms, fuel lines etc. can be fastened.
The fastening elements and/or the workpieces, in this case, are frequently contaminated, for example by solid lubricant and/or deep-drawing oil, in particular as residue from previously carried out cold working processes. To clean the fastening element or workpiece, it is known from the prior art to conduct a cleaning process prior to the actual stud welding process. In the case of said cleaning process, prior to the generating of the welding arc at a relatively high current intensity, a cleaning arc, which has a lower current intensity, is generated between the workpiece and the fastening element to be welded on. In the case of said cleaning step, the welding surfaces located opposite each other, as a rule, are not yet melted. In this connection, on account of the emission of electrons, the component with negative polarity experiences a certain cleaning temperature rise which results in the combustion of the wetting substances. The residue of said combustion process is then deposited on the component with positive polarity if it does not immediately evaporate. By frequently changing polarity, the surfaces can consequently be cleaned well.
In the case of very thin-walled workpieces with sheet thicknesses of, for example, less than or equal to 1 mm, there is the risk of frequent polarity changes leading to “root penetration”, where the workpiece in the region of the welding surface is melted not only on the surface but directly through such that a hole is generated at this position and a stud weld connection is no longer possible.
Said problem is produced in particular when aluminium fastening elements or workpieces are used.
In the prior art, a stud welding method with a pre-conducted cleaning process is described in document EP 1 183 127 B1.
In addition, cleaning metal workpiece surfaces in an inductive manner is known, for example see DE 10 2008 028 272 A1.
Document DE 10 2007 032 067 A1 also makes known a method for cleaning a workpiece surface by means of a cryogenic medium.
Document DE 10 2009 053 762 A1 proposes a cleaning apparatus for a stud welding head, said cleaning apparatus having a pair of brushes.
Dry cleaning, for example by means of acetone, is also known in the prior art.